Primary fibromyalgia may involve an anomaly in the metabolism of serotonin responsible for the sleep disorders and diffuse pain. Effectiveness of an agent with pure serotonin-agonist properties (fluoxetin hydrochloride) was evaluated in 23 patients during a three-month open study. Treatment had no effect on pain severity, number of tender sites, or pain score. Sleep disorders improved and 57% of patients believed the treatment was effective. Adverse events were recorded in 43.4% of patients, with the most common being nausea (21.7%). Effectiveness and tolerance of fluoxetin hydrochloride in fibromyalgia are mediocre. A double-blind placebo-controlled trail versus a placebo is needed to clarify these preliminary findings.